Our work is directed towards the precise definition of the ribonucleoprotein structure of the transcript of the mouse beta major globin sequence and the development of a meaningful assay for this structure. In order to accomplish this, we are mapping the position in the transcript of nucleotide sequences which are accessible to, or protected from, exogenous nuclease. This is being done by using as probes specific restriction fragments of the mouse beta major globin cloned gene labeled to high specific activity.